There are known stereoscopic image display apparatuses allowing viewers to view stereoscopic images with the naked eyes without wearing special glasses. Such a stereoscopic image display apparatus displays a plurality of images from different viewpoints, and controls light beams for the images with an optical element. The controlled light beams are guided to both eyes of a viewer, who can then recognize a stereoscopic image if the viewer is at a proper viewing position. There is known an optical element using, as the optical element, a parallax barrier or a lenticular lens.
However, in the apparatus using the parallax barrier or the lenticular lens as the optical element, the resolution of the stereoscopic image or the display quality of a planar (2D) image may be degraded. In order to keep the display quality of the planar image, there is known a technique using a liquid crystal optical element or a birefringent element as an optical element capable of switching a display between a 2D display and a 3D display. However, a switching speed of switching from the 3D display to the 2D display often becomes an issue. As a technique to increase the switching speed, there is known a technique of establishing an intermediate voltage state when switching the display from the 3D display to the 2D display in a multielectrode structure in which electrodes formed on an upper substrate and electrodes formed on a lower substrate are arranged to face each other. In switching the display from the 3D display to the 2D display in this technique, a voltage state is set from a first voltage state to the intermediate voltage state. In the first voltage state, a voltage applied to each electrode is controlled in such a way that a refractive index distribution is achieved which acts as a lens array in which a lens is periodically arranged. In the intermediate voltage state, a voltage V1 applied to electrodes corresponding to the ends of the lens formed in the first voltage state and 0 V voltage are alternately applied to each electrode. Thereafter, a voltage state is set to a second voltage state in which a voltage applied to each electrode is controlled in such a way that a refractive index distribution with a fixed refractive index is achieved.
In the aforementioned related art, however, the voltage applied to first-type electrodes corresponding to the ends of the lens is the maximum among voltages applied to the electrodes in the first voltage state, thereby likely causing misorientation at the ends of the lens. Moreover, the voltage applied to the first-type electrodes in the intermediate voltage state is set to the same value (V1) as the voltage applied to the first-type electrodes in the first voltage state, whereby it has been difficult to sufficiently increase the switching speed when switching from the refractive index distribution corresponding to the first voltage state to the refractive index distribution corresponding to the second voltage state.